In manufacturing processes, multiple structural components are often manufactured separately and then assembled together. For example, tens, or even hundreds (or more) of structural components may be assembled together to form structures in aerospace or aviation applications, such as in the manufacture of aircraft wings. Gaps between such structural components may result from manufacturing tolerances (expected, but unplanned variations) of the components, and/or from unique challenges associated with certain materials. For example, in making composite parts, geometric variations in final parts may result from variations in fiber diameter and/or variations in resin volume, which may accumulate via a plurality of layers of material that are laid up to form the composite part.
An example where such issues arise is in the assembly of a skin panel to rib feet to form a wing-box of an aircraft. FIG. 1 shows an example of a wing box structure for an aircraft wing 10. Generally, aircraft wing 10 includes a ladder-like structure formed by a plurality of ribs 12 spaced apart between one or more longer spars 14. Ribs 12 generally define the overall shape of aircraft wing 10, with a skin panel 16 (partially shown in dashed line, also referred to as a wing skin) being attached to ribs 12, conforming to the shape of the ribs 12. In a conventional process, skin panel 16 is manufactured in a desired shape and is then brought into position to engage ribs 12 and spars 14. The underlying rib/spar ladder structure may have, for example, outwardly projecting rib feet on which respective parts of the inner surface of the skin panel 16 are intended to rest so that fasteners can be inserted through the skin panel 16 and the rib feet to secure the skin panel to the ladder structure. Engineering tolerances for the skin panel 16 are generally greater than is the acceptable error in fit between the skin panel 16 and the ladder structure, which can result in interface gaps between the skin panel 16 and the ladder structure in certain areas. For example, when the skin panel is brought in an unstressed state into position against the rib feet, it is commonly found that, whilst some rib feet are in contact with the skin panel, others are spaced from it. The skin panel can only be deformed a slight amount to safely accommodate these gaps without creating stresses stored within the skin panel or affecting the shape and aerodynamics of the skin panel. However, enforcing extreme tolerances increases costs and/or may simply be infeasible. Thus, manufactures typically rely on filling the gaps by applying a liquid or solid shim between ribs 12 and skin panel 16, where needed (e.g., in the positions of the gaps).
To create the needed shims, the skin panel and the underlying structure of the wing-box are generally brought into position next to each other so that gaps between the rib feet and the inner surface of the skin panel can be measured. Shims are then made to fill the gaps, the shape and size of each shim being chosen according to the shape and size of the respective gap to be filled. In this approach, the final assembly of the skin panel and the underlying structure of the wing-box is deferred until after the shims have been manufactured, at which point the skin panel and the underlying structure are brought into position next to each other again. Such added steps and delays increase manufacturing costs and decrease efficiency. Additionally, placement of these shims is a generally time-consuming and expensive process. In some cases, composite structures may have to be assembled and disassembled several times to measure the shim gaps and drill and clean holes. Care also has to be taken to ensure that a shim that has been manufactured to a particular size and shape to fill a particular gap is used to fill the correct gap, and not inadvertently used to fill a different gap.
In some manufacturing processes, even further delays are introduced at other stages. For example, efforts have been made to reduce the use of shims, such as by fabricating the skin panel and then custom-manufacturing the spars and spar caps, once the as-built dimensions of the skin panel are known. This method can create an inventory build-up of wing skin panels while the spars and spar caps are being made. The wing skin panels are large structures that must be held and supported properly while the spars and spar caps are made, to prevent damage to the skin panel, which takes up valuable warehouse space for long periods of time. And even such attempts may still require the use of shims in some places.